Offshore positioning system and method

ABSTRACT

A system for measuring the attitude of an object in a fixed frame of reference from a moving frame of reference, comprising a first imaging device for producing image data for the object from a first direction, a second imaging device for producing image data for the object from a second direction having a component perpendicular to the first direction and an attitude sensing arrangement for determining an attitude of the first and second imaging devices with respect to the fixed frame of reference at the time the respective images are taken and for generating attitude data. An image processing system analyses and combines the respective image data and attitude data to determine the attitude of the object. The system is particularly useful for determining attitude of offshore piles during piling operations or for monitoring the departure angles of pipes and cables during laying thereof.

BACKGROUND Field of the Invention

The present invention relates to devices and systems for determining anattitude of an object in a fixed frame of reference by observations froma moving frame of reference and more particularly to a system capable ofaccurately measuring the verticality of a pile or similar object from afloating platform or vessel. The invention also relates to a method ofperforming such measurements.

Description of the Related Art

Engineering operations at sea often involve considerable difficulty dueto the constant movement of the waves. This is especially the case whenaccuracy is required in performing measurements. For a vessel at sea,measurements may be made in a fixed frame of reference relative to theearth or in a moving frame of reference relative to the vessel.

One situation where measurements in the absolute frame of reference arerelevant is in the installation of fixed structures from a floatingplatform or vessel. In recent years large numbers of offshore structureshave been installed on or in the sea bed, including wind turbines,drilling platforms, bridges, cables, pipelines and the like. Theinstallation work is generally performed from a suitable barge or vesselusing cranes and other lifting facilities. In the case of wind turbines,a single pile may be driven into the seabed using a hydraulic hammer orother pile-driving facility mounted on a heavy lift vessel. Thesemonopiles should be installed vertically and it is essential that duringdriving the verticality is continuously monitored and, where necessary,corrected. At present, the procedure for performing such measurementsinvolves a human operator approaching the monopile and performing amanual measurement of the structure using a handheld inclinometer. Thismeasurement thus takes place directly in the fixed frame of reference ofthe monopile. During measurement, the pile-driving must be stopped.After evaluation of the results, the crane operator must be informed ofthe results and instructed with respect to the required correctiveaction.

In the case of laying of cable from a cable lay vessel, it may bedesirable to closely monitor the angle of deployment of the cable inorder to determine its trajectory to the seabed. In conventional systemsthis may be achieved by locating an angle sensor against the cable atthe departure location. The sensor may be a pitch and roll type anglesensor which requires direct contact with the cable for operation.Positioning of such sensors can be difficult and they may requirefrequent attention e.g. on passing joints on the cable. They may also beeasily damaged, especially when laying multiple cables and at presentare not applicable to all situations. Similar considerations applyduring pipe-laying.

It would be desirable to provide a system that simplified suchmeasurements and could be implemented in real time without interruptingoperations.

BRIEF SUMMARY OF THE INVENTION

According to the invention there is provided a system for measuring anattitude of an object in a fixed frame of reference from a moving frameof reference, the system comprising: a first imaging device forproducing image data for the object from a first direction; a secondimaging device for producing image data for the object from a seconddirection having a component perpendicular to the first direction; anattitude sensing arrangement for determining an attitude of the firstand second imaging devices with respect to the fixed frame of referenceat the time the respective images are taken and for generating attitudedata; and an image processing system for analyzing and combining therespective image data and attitude data to determine the attitude of theobject. By the use of imaging devices, observations may be made at adistance from the object to be measured. Measurements may thus takeplace without requiring anyone to be in the direct vicinity of theobject. The measurements may be taken from any distance. Preferably, thedistance is between 0.5 meters and 100 meters, more preferably between 5meters and 30 meters. In most cases, the first direction and the seconddirection will be generally perpendicular to each other, although thereis usually no requirement of exact perpendicularity. In the presentcontext, attitude is intended to refer to the orientation of acoordinate frame attached to the object with respect to another frame,such as a geodetic reference frame or an earth-fixed frame associatedwith a local direction of gravity, and may be expressed in terms of thedegrees of deviation with respect to the reference axis. In general,attitude will be understood to cover three degrees of freedom, namelypitch, roll and yaw. Nevertheless, it will be understood that in certaincircumstances, just two degrees of freedom such as pitch and roll may bemonitored within a defined body frame having an arbitrary heading suchas the vessel reference frame. Alternatively, inclination andinclination direction may be monitored, for instance for an uprightobject with axial symmetry.

The attitude sensing arrangement may be any suitable device capable ofrecording the local orientation or attitude of the respective imagedevice with respect to the fixed reference frame. In many cases suchdevices are referred to as inertial measurement units (IMU) but this isnot intended to be restrictive on any particular principle of operation.In particular, the attitude sensing arrangement may be based onmechanical gyroscopic action, fiber optic gyroscopes (FOG), laser ringgyroscopes (LRG), MEMS based technologies, accelerometers,magnetometers, pendulum, electrolytic inclinometers, liquid capacitiveinclinometers or any combination of the above. Most preferably, theattitude sensing arrangement also comprises a heading indicator allowingmomentary determination of the attitude of the imaging devices withrespect to the earth, in particular, geodetic or magnetic north. Theattitude sensing arrangement should preferably have a dynamic accuracyof better than 1.0°, more preferably better than 0.5° and mostpreferably better than 0.2°, at least in the pitch and roll directions.

In a simple embodiment, the attitude sensing arrangement may comprise asingle attitude sensor for both imaging devices. This single devicecould be a pitch and roll sensor calibrated to the vessel or could be athree axis attitude sensor with north-seeking capability. In both thesecases, the positions of the first and second imaging devices must remainfixed relative to each other within the moving frame of reference frominitial set-up. Such a system may be used in combination with a ship'sIMU or attitude sensor, whereby the first and second imaging devices arelocated at fixed locations. In a more preferred embodiment, first andsecond attitude sensors are provided, each integrated with a respectiveimaging device for movement therewith. Each local attitude sensor may bea non-north seeking attitude sensor whereby readings within a vesselframe of reference may be determined if the position of the imagingdevice relative to the vessel is accurately established. It will also beappreciated that each local attitude sensor could even be a single axisattitude sensor, whereby more constraints would be required on thepositioning of the imaging devices and on the resulting attitudedetermination. The system has considerable additional advantage if eachof the attitude sensors is a 3-axis attitude sensor with north-seekingcapability. In this case, each imaging device can be relocated togetherwith its attitude sensor and can instantly commence measurement, sinceit automatically knows its own heading. Since the heading of theobservation is now defined in the geodetic reference frame, it allowspitch and roll to be defined and computed for any arbitrary absoluteheading. This heading could be e.g. North or, when the vessel heading isknown, the heading of the vessel. This allows considerable versatilitysince an imaging device can easily be repositioned if its view isobscured or if it is otherwise inconvenienced. As a further consequenceof the dedicated 3-axis north-seeking attitude sensor, each imaging unitmay in fact be located in its own frame of reference, movingindependently of each other. It will thus be understood that they mayeven be located on different vessels or that theoretically one or bothof the imaging devices may be portable and hand held during measurement.

According to an important aspect of the invention, the system furthercomprises a clock, arranged to time stamp the attitude data. In order toeffectively process the image data and the attitude data, there must beclose correlation in time between the respective measurements. Foroperation on board a moving vessel, the attitude may change very rapidlyand the attitude data should preferably be time stamped to an accuracyof at least 1 ms. Most preferably, the time stamp will be accurate toaround 0.1 ms. The required timing accuracy, including unaccountedprocessing and communication latencies, may be expressed as a functionof the required measurement precision and the expected movement rate ofthe moving frame. For example on board a vessel, it may be desirablethat the timing accuracy should be better than the required pitch androll measurement accuracy divided by the maximum expected angular rateof the vessel. The required relative timing accuracy between the firstand second imaging devices may be less critical, especially if anexpected angular rate of movement of the object is very low compared tovariations in the moving reference frame. Even the difference betweencapture time and transmission time may need to be taken into account inorder to ensure that the correct data items are associated to oneanother. The clock may be a local clock applicable to the attitudesensor and its associated imaging device or devices. Most preferably,the clock is linked to a global timing system such as GPS, which allowsreadings to be correlated with other events such as a global positionindication. Preferably the clock also issues a clock signal forsynchronizing a capture time of the image data. The skilled person willbe well aware of various options for ensuring that the image data andattitude data are accurately synchronized to such a clock signal.

As discussed above, the invention is particularly applicable formounting aboard a vessel. In that case, according to a preferredembodiment of the invention, the attitude of the object may be givenwith respect to a heading of the vessel. For a vertical object such as apile, the attitude will preferably be the inclination of the pile andits direction. The direction of inclination of the top of the pile withrespect to the base can then be displayed relative to the bow of thevessel. For a crane or pile driver mounted to and moving with thevessel, corrective action may be more easily taken in this localreference frame. It is however not excluded that the attitude of theobject is also or alternatively given with respect to a geodeticcoordinate frame.

In an alternative configuration, the device may be used for monitoring apipe or cable laying operation by monitoring the direction of departureof a pipe or cable string from a vessel. The attitude of the string atdeparture can be used to compute the trajectory of the string to theseabed. Of particular interest is the angle of the string with respectto the vertical (or horizontal direction). The departure heading may bedetermined based on position data for the vessel and the point ofcontact with the seabed, although it is not excluded that the heading ofthe string could also be determined by the claimed device.

According to a yet further advantage of the invention, the system alsocomprises a calibration facility. This may be arranged to calibrate thetotal body-frame roll axis error of a respective imaging device and theattitude sensing arrangement. In this context, total body-frame roll isintended to refer to the combined error due to both the attitude sensoraccuracy and any misalignment between the mounting of the camera andattitude sensor. This may be accomplished by taking an image of thehorizon, which by definition must be horizontal. Frequent self-checkscan be carried out, which have the advantage that a device can beverified to work correctly in the marine environment. Distortionsarising from e.g. shipment of the unit, resulting in increasedmeasurement errors, can be easily detected using such a method.

According to a still further aspect of the invention, the system allowsmeasurement and determination of the attitude in real time. There mayalso be an attitude display providing a real-time representation of theattitude of the object. One particularly convenient display is in theform of a sight glass or spirit level, allowing an operator to monitordeviation of the object from a desired attitude. The system may also oralternatively comprise a user interface and an image display. In oneconvenient form, the image processing system may be arranged such that auser can manually pick a portion of an image of the object on thedisplay for use in determining the attitude of the object. The portionof the object may be a characteristic edge or marking on the object andpicking may take place using a mouse or similar pointer to identify theportion on the image display. Additional software may be provided forlocking the image based on image recognition of the characteristics ofthe portion. This can be of use in situations where the object becomespartially obscured and a human operator may more reliably identify therelevant portions.

Although the system may be used for determining the orientation orattitude of many alternative objects, it is particularly suitable foruse in determining the verticality of a pile, more specifically the sortof pile or monopile used for offshore wind turbine generators. In oneembodiment of the system, the image processing system is arranged to:determine left and right borders of the pile from the first imagingdevice and identify a location of the plane therebetween that passesthrough the first imaging device; determine left and right borders ofthe pile from the second imaging device and identify a location of theplane therebetween that passes through the second imaging device; andcombine the location of the planes from both the first and secondimaging devices to determine the attitude of the axis of the pile.Assuming that the pile is rotationally symmetric, the mid-plane betweenthe edges will identify the central axis irrespective of the directionof viewing. This makes it considerably easier to evaluate the results,as it is independent of the distance between the imaging devices and thepile. It is also applicable even when the pile tapers.

The invention also relates to a device for measuring an attitude of anobject in a fixed frame of reference from a moving frame of reference,the device comprising: an imaging device for producing image data forthe object; an attitude sensor for determining an attitude of theimaging device with respect to the fixed frame of reference at the timethe image is taken and for generating attitude data; and a communicationport for transmitting data to a data processing system for determiningor displaying the attitude of the object. In a simple form, the attitudesensor may be a one- or two-axis attitude sensor without north-seekingcapability. It will be understood that north-seeking devices areconsiderably more expensive and a simple non north-seeking device may bemore than adequate for many purposes, especially when used e.g. incombination with a ship's north-seeking facility or with additional GPSpositioning. A more complex embodiment of the invention provides asingle device in which the imaging device and a north-seeking attitudesensor are integrated. Such a device is extremely versatile and can infact be moved to a given location and immediately provide relevant inputfor determining the attitude of the object. This may be the case, evenwhen the absolute position is unknown, based merely on the attitudeinformation. It will be understood that the data transmitted may be theimage data and the attitude data. In this case, the remote dataprocessing system may include an image processing system for analyzingand combining the image data and attitude data to determine the attitudeof the object as described above. Alternatively, an image processingsystem may be provided in the device and the transmitted data maycomprise partially processed data related to the attitude of the object.The device may also include a local image display and user interface.

According to an embodiment, the device may comprise a clock, arranged totime stamp the image data relative to the attitude data. The clock maybe a local clock or may be a clock signal derived from an externalsource such as the data processing system or a satellite navigationsystem. Stamping the image and attitude data in an accurate manner is ofimportance in ensuring that they are coherent, especially when analysistakes place subsequently. The clock or time stamp facility may be partof a real-time signal multiplexer that combines the attitude data andthe image data.

Most preferably, the device is portable and the imaging device and theattitude sensor are fixed to each other to prevent relative movement. Byportable, it is understood that the device is of a size and weight to beeasily carried and set up by one person. It will be understood that sucha device may even be used alone to provide measurements from twodirections by moving from a first position to a second position. Such adevice could be mounted on a separate vessel, movable with respect tothe object being measured.

It will be further understood that various imaging devices may providethe desired function. These may include laser imaging devices, infra-redimaging or conventional camera based imaging devices. Most preferably,the imaging device is a camera having automatic iris control, allowinguse in varying lighting conditions. It will be understood that,throughout the day, the position of the sun will vary and it may beundesirable to shift the imaging device when the lighting condition isbad. Automatic lighting control can mitigate this difficulty, allowingthe operator to continue measurements from the same location. Additionaldesirable characteristics of the camera include anti-smearing andanti-blooming correction to further improve image quality.

In a still further embodiment, the device further comprises acalibration facility arranged to calibrate the attitude sensor fordetermining an attitude of the imaging device with respect to thehorizon. The mounting of the attitude sensor to the imaging device ispreferably robust. Nevertheless, when using the device for extendedperiods or in the event that it has been subject to shock, it may bedesirable to have a simple self test possibility. This may be achievedby calibrating it against the horizon, which, for offshore locations,provides an accurate horizontal axis. The calibration facility may bepartially local and partially remote e.g. with an image processingsystem being used to identify the horizon in the same way asidentification of the object takes place.

According to a further desirable embodiment, the device may alsocomprise a satellite positioning system providing position data for theimaging device. Although it is not directly required to know the exactlocation of the device in order to determine attitude of the object,additional benefits may be achieved by knowing location. It will beunderstood that position data for the device in combination withposition data for the object could also be used to provide the headingrequirement of the device. The satellite positioning system may be anyGNSS (e.g. GPS, Glonass or both) or based on any other system capable ofreliably determining position.

The invention also relates to a vessel comprising the system or thedevice as defined above. The vessel may be any vessel, including but notlimited to, a heavy lift barge performing pile driving operations, acable or pipe laying vessel, an ROV or a support vessel to one or moresuch vessels.

The invention further relates to a method of determining an attitude ofan object in a fixed frame of reference based on observations from amoving frame of reference and a computer program product forimplementing the method. The method may comprise: viewing the objectfrom a moving frame of reference in a first direction to produce a firstimage data set; viewing the object from a moving frame of reference in asecond direction to produce a second image data set; collecting attitudedata representing a relative attitude of the moving frame of referencewith respect to the fixed frame of reference for each image data set;and analyzing and combining the respective image data and attitude datato determine the attitude of the object. The method may be carried outusing the system or device as described above or hereinafter and isparticularly directed to determining the verticality of an offshore pileduring driving thereof or of a cable or pipe during an offshore layingoperation.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention will be appreciated uponreference to the following drawings of a number of exemplaryembodiments, in which:

FIG. 1 shows a schematic view of a first embodiment of the presentinvention;

FIGS. 2A and 2B show images taken by the first and second cameras of theembodiment of FIG. 1;

FIGS. 2C and 2D show images taken by the first and second cameras of atapered monopile in an alternative embodiment;

FIG. 2E shows an attitude display of a monopile during use of theinvention of FIG. 1;

FIG. 3 shows a device and system according to a second embodiment of theinvention;

FIG. 4 shows a device according to the invention in use on a cable-layvessel; and

FIG. 5 shows a schematic view of the orientation of the cable of FIG. 4

DETAILED DESCRIPTION

FIG. 1 shows a schematic view of a system 1 for measuring the attitudeof a monopile 2 according to a first embodiment of the invention. Thesystem 1 is mounted upon a barge 4, which is being used for installationof the monopile 2. Cranes and pile-driving equipment is not shown forthe sake of convenience but may be otherwise conventional.

The system 1 comprises a first camera 6 and a second camera 8 mounted onthe barge 4 within view of the monopile 2 and directed towards it. Thecameras 6, 8 are mounted to view the monopile from orthogonal directionsi.e. the lines of sight from the respective camera to the monopile areperpendicular to each other. As will be understood by the skilled personin the following, these directions need not be perfectly perpendicular.In the present embodiment, the first camera 6 is aimed along a left handedge of the monopile 2 and is aligned across the barge 4 in a directiondenoted X. The second camera 8 is aimed at a right hand edge of themonopile and directed in the direction denoted Y with respect to theframe of reference of the barge 4. Also aboard the barge 4 are a 3-axisnorth-seeking attitude sensor 10 and an image processing system 12including an image display 14 and a user interface 16. The imageprocessing system 12 is in fact a standard computer running dedicatedsoftware.

FIG. 2A is an image taken by the first camera 6 at a time T1 as viewedon the image display 14. It shows the monopile 2 and an indication ofthe Z and Y directions, together with an indication of the time T1. TheZ direction is the vertical direction within the moving frame ofreference of the barge 4. V is the true vertical within the geodeticcoordinate frame. The image display 14 also includes pointers 18A, Bthat can be manipulated through the user interface 16, e.g. by use of amouse or the like.

FIG. 2B is an image taken by the second camera 8 at a time T2 as viewedon the image display 14. It shows the monopile 2 and an indication ofthe Z (local-vertical), V (geodetic vertical) and X directions, togetherwith an indication of the time T2. Also shown are pointers 19A, B.

FIGS. 2C-2D show images taken by the first and second camera 6, 8, inanother case wherein the imaged monopile 3 has a slightly tapered shape.Here, the tapered pile 3 has a truncated conical outer pile surface forwhich a radial distance from a nominal central axis A of the pile 3changes slowly and linearly as a function of axial distance along theaxis A. The tapered shape of the outer pile surface may be described bya non-zero taper angle γ relative to the axis A.

FIG. 2E shows an attitude display 20 in a single spirit-levelrepresentation of the attitude of the monopile 2. A bubble 22 indicatesthe position of the monopile with respect to the vertical V in the fixedreference frame. The attitude display 20 shows the magnetic or geodeticNorth direction N, giving attitude in the fixed reference frame and canalso show the X and Y directions allowing the attitude of the monopile 2to be seen within the local frame of reference. The attitude display 20may be provided on the crane operator or pile driver's display and mayalso be displayed on the image display 14.

Operation of the system 1 will now be described with reference to FIGS.1 and 2A, 2B. In use, once the monopile 2 has been placed at the correctlocation and driving has commenced, it is effectively fixed within thegeodetic frame of reference of the earth. It still moves slowly due tothe driving operation but this slow movement may be ignored for thefurther discussion. Images are taken and displayed on the image display14 as shown in FIGS. 2A and 2B with an update frequency, e.g., of around1 Hz. A user operating the system 1 uses the user interface 16 toidentify two points on the left hand edge of the monopile 2 in FIG. 2Ausing pointers 18A and 18B. The image processing system 12 accuratelyidentifies the chosen edge and locks onto it using an edge detectionsoftware module. Such a module is generally conventional softwarecapable of pixel analysis to determine an edge of an object and is notfurther discussed here. Once the edge is detected, the image processingsystem 12 can accurately represent the position of this edge within theZY frame of reference of the barge 4 for the time T1. Thereafter, theimage processing system 12 can continue to follow the edge withoutrequiring reselection, unless the image should become obscured for somereason. The image processing system 12 also receives attitude data fromthe attitude sensor 10 which is time-stamped as having been measured attime T3. The attitude data is sampled at a rate of e.g. 100 Hz andinterpolation is used between these values to match the image data attimes T1 and T2 to the attitude data at time T3. This allows anevaluation of the offset of the true vertical V with respect to thelocal vertical Z at the time T1. The same is repeated for the image ofthe second camera 8 taken at time T2 as shown in FIG. 2B.

As illustrated in FIGS. 2A-D, the system 1 may operate in asingle-edge-per-camera mode, wherein it determines the inclination ofthe monopile while detecting and tracking just one of the two pile edgesthat are visible in each camera image. In single-edge-per-camera mode,the system 1 simultaneously tracks two pile edges, one edge selectedfrom each one of the two camera images that are (quasi-)synchronouslyacquired from the two (approximately) orthogonal viewing directions Xand Y. For a cylindrical pile 2 (FIGS. 2A-B), the system 1 is configuredto determine and track the attitude of the pile 2 based on a positionand orientation of the border plane intersecting the two points 18A, 18Bon a left hand edge of the monopile 2 in FIG. 2A, as well as on theposition and orientation of the further border plane intersecting thetwo points 19A, 19B on a right hand (trailing) edge of the monopile 2 inFIG. 2B. For the tapered pile 3 (FIGS. 2C-D), the taper angle γ of theradial outer pile surface relative to the central pile axis A should besupplied to the system 1 in advance. In this case, the system 1 isconfigured to determine the attitude (e.g. inclination and inclinationdirection) of a central axis A of the pile 3 based on the positions andorientations of the border planes corresponding with the points 18A,18B, 19A, 19B along the edges of the monopile 3, and on thepredetermined value for the taper angle γ. In cases wherein the non-zerotaper angle γ for the pile 3 is smaller than the pile orientation anglemeasurement accuracy achievable by the system 1, it may be preferred tolet the system 1 rely on a cylindrical pile model (by inputting γ=0°) inorder to simplify calculations.

Once both images have been analyzed, the results may be combined in thesingle spirit-level representation of the attitude display 20 as shownin FIG. 2E. As the first and second cameras 6, 8 track the monopile, theposition of the bubble 22 changes in real time and the engineer orsurveyor can give directions for corrective action to the operator ofthe pile driver.

In the first embodiment of FIG. 1, a single attitude sensor is used,which is the ship's own IMU. The cameras 6, 8 are high resolutiondigital cameras operating according to GigE vision protocol and havingautomatic iris control and anti-smearing and anti-blooming correction.This allows them to compensate for changing light conditions.Nevertheless, movement of the cameras 6, 8 with respect to the barge isundesirable, as this would require significant recalibration in order todefine their relative positions in the local reference frame.Consequently, such an arrangement may be unsuitable where the locationof the monopile 2 with respect to the barge 4 may change e.g. from oneoperation to the next or where significant chance of obstruction of oneof the cameras is present.

A second embodiment of the invention is shown in FIG. 3, in which likeelements to the first embodiment are denoted with similar referencenumerals preceded by 100. The device 101 of FIG. 3 comprises a camera106 and 3-axis north-seeking attitude sensor 110 integrated together ina single portable unit. This means that relative movement between thecamera 106 and attitude sensor 110 is prevented. Furthermore, since thedevice 101 includes its own attitude sensor 110, the momentary attitudeof the camera 106 in the geodetic reference frame can be preciselymonitored. The device 101 also includes a 1 GB Ethernet switch 126 and atime stamp generator 128. The device 101 communicates through switch 126with a data processing system 130 which has its own communications port132. It will be understood that although an Ethernet connection isshown, communication may also take place by wireless means. The dataprocessing system 130 includes an image processing system 112, imagedisplay 114 and user interface 116. According to the second embodimentof the invention, the image processing system 112 operates slightlydifferently to that of the first embodiment in that it identifies leftand right edges of the monopile 102. This may be done automatically orwith manual pointers as in the first embodiment. Once these edges asobserved in the images are determined, the image processing system 112calculates a plane through the centerline CL of the monopile and thecamera center by determining the middle between the planes definedbetween the camera and the two observed edges. Assuming a surface ofrevolution, this will be true, irrespective of whether the monopile 102varies in width and leads to greater result accuracy. Together with theimage data from the camera 106, the data processing system 130 alsoreceives attitude data from the attitude sensor. Since this includesheading data as well as data related to pitch and roll, the imageprocessing system 112 can use this data to determine the precisedirection from which the image has been made and orientate thecenterline CL accordingly. The attitude data is time-stamped by the timestamp generator 128 which also issues a clock signal for the camera 106to synchronize the image data acquisition. The image data and attitudedata are together communicated via the 1 GB Ethernet switch 126 to thedata processing system 130. FIG. 3 also shows a second device 101B whichprovides data for determining the 3D orientation of the centerline CLfrom another direction. Based on the two readings, the data processingsystem 130 can determine the attitude of the centerline and display itas described above and shown in FIG. 2E and as an attitude display 120on the image display 114.

As mentioned above, the device 101 is portable and can be moved to alocation from which the required image is to be taken. Since theattitude sensor 110 provides real time attitude data, it may even behand-held. In order to ensure that the device 101 is correctlycalibrated to the geodetic reference frame, it further includes a selfcalibration button 134. Operation of the self calibration button 134requires the camera 106 to be directed towards an open expanse ofhorizon. Activation of the self calibration button 134 generates animage of the horizon H and correspondingly time-stamped attitude data.The image processing system 112 identifies the horizon H eitherautomatically or with the assistance of an operator and compares theviewed horizontal with the value for horizontal measured by the attitudesensor and transmitted as attitude data. If there is a discrepancy, theimage display indicates the difference and provides an offset to thedevice 101 for all further calculations. If the offset is too great, theoperator is warned that the device may be faulty.

The embodiment of FIG. 3 has been described for use in a system in whichtwo devices 101, 101B provide attitude data which is combined todetermine in real time the attitude of an object. It is also possible todetermine the attitude in quasi-real time using a single device 101. Inthat case, the device 101 is moved around the monopile, either by beingmoved around on the deck of the barge or by being moved around themonopile aboard a support vessel. In that case, the image data andattitude data can be supplied intermittently to the data processingsystem 130 which updates the attitude display 120 as new angles of vieware provided. This can be sufficiently accurate if the monopile isdriven relatively slowly compared with the change in position of thedevice 101. The data processing system 130 may provide a suitable alarmif insufficient data is provided from a certain direction within a giventime.

An alternative embodiment of the invention is shown in FIG. 4, for usein determining the angle of departure of a cable during a cable-layoperation. Like elements to the first embodiment are provided withsimilar references preceded by 200.

According to FIG. 4, cable-lay vessel 204 is provided at its stern witha stinger or chute 211 over which cable 202 is deployed to the seabed.Attitude measuring devices 201 according to the invention are mountedoverboard at the stern. The attitude measuring devices 201 are similarto the devices 101 of the second embodiment except that they incorporateattitude sensors without north-seeking capability. Cameras 206 aredirected towards the cable 202 at the point where this leaves the chute211. Data acquisition takes place as in the previous embodiments, withimage data and attitude data being time stamped and processed todetermine the attitude of the cable 202 at its point of departure fromthe vessel 204 in the geodetic frame of reference. FIG. 5 is a schematicview of the cable 202 at this point, indicating the geodetic verticaldirection V and the horizontal plane H. The angle of the cable 202 withrespect to its orthogonal projection onto the horizontal plane H isgiven by α. In this configuration, the actual heading of the cable 202is not required, since this may be determined by other data, includingthe location of the vessel and the touchdown point of the cable at theseabed.

Thus, the invention has been described by reference to certainembodiments discussed above. It will be recognized that theseembodiments are susceptible to various modifications and alternativeforms well known to those of skill in the art. In particular, while theabove techniques have been described in the context of driving monopilesfor offshore wind turbine generators and cable laying, the invention mayalso be applied to other offshore structures, notably but not limited tooffshore oil and gas installations, underwater seabed structures asfoundation piles, oil well templates, underwater pipelines, pipe-laying,and underwater installation equipment such as frames and liftingfacilities. Many modifications in addition to those described above maybe made to the structures and techniques described herein withoutdeparting from the spirit and scope of the invention. Accordingly,although specific embodiments have been described, these are examplesonly and are not limiting upon the scope of the invention.

The invention claimed is:
 1. A system for measuring in real time anattitude of an object in a fixed frame of reference relative to earthfrom a moving frame of reference relative to a vessel, the systemcomprising: a first imaging device configured to acquire a plurality offirst images of the object at subsequent times and from a firstdirection in the moving frame of reference relative to the vessel; asecond imaging device configured to acquire a plurality of second imagesof the object at subsequent times and from a second direction in themoving frame of reference relative to the vessel, the second directionhaving a component perpendicular to the first direction; an attitudesensing arrangement configured to measure momentary attitudes of thefirst and second imaging devices with respect to the fixed frame ofreference relative to earth during a period in which the first andsecond pluralities of images are acquired and configured to generateattitude data; and an image processing system configured to analyze andcombine the respective pluralities of image and attitude data todetermine the real time attitude of the object in the fixed frame ofreference relative to earth.
 2. The system according to claim 1, whereinthe attitude sensing arrangement comprises a three-axis attitude sensorwith heading indicator.
 3. The system according to claim 1, wherein theattitude sensing arrangement comprises first and second attitudesensors, each integrated with a respective imaging device for movementtherewith in the moving frame of reference.
 4. The system according toclaim 1, wherein the first and second imaging devices are mounted aboardthe vessel and the attitude of the object is given with respect to aheading of the vessel.
 5. The system according to claim 1, wherein thefirst and second imaging devices are mounted aboard the vessel and theattitude of the object is given with respect to a geodetic coordinateframe.
 6. The system according to claim 1, further comprising anattitude display providing a real-time representation of the attitude ofthe object.
 7. The system according to claim 1, further comprising auser interface and an image display and the image processing system isconfigured to enable a user to manually pick a portion of an image ofthe object on the image display for use in determining the attitude ofthe object.
 8. The system according to claim 1, further configured todetermine, via the image processing system, verticality of a pile havinga central axis, wherein the image processing system is furtherconfigured to: determine left and right border planes of the pile fromthe first imaging device and identify a location of a first planepassing through the first imaging device and being equidistant from theobserved left and right border planes; determine left and right borderplanes of the pile from the second imaging device and identify alocation of a second plane passing through the second imaging device andbeing equidistant from the observed left and right border planes;identify an intersection of the first and second planes observed fromboth the first and second imaging devices to determine the attitude ofthe central axis of the pile.
 9. The system according to claim 1,further configured to determine, via the image processing system,verticality of a pile having a central axis, wherein the imageprocessing system is further configured to: determine a border plane ofthe pile from a first image acquired by the first imaging device;determine a further border plane of the pile from a second imageacquired by the second imaging device; determine the attitude of thecentral axis of the pile based on positions and orientations of theborder plane and the further border plane.
 10. The system according toclaim 9, wherein the image processing system is further configured to:determine only one border plane of the pile in the image data from thefirst imaging device, using a single edge of the pile visible in atleast one image acquired by the first imaging device; determine only onefurther border plane of the pile in the image data from the secondimaging device, using a single further edge of the pile visible in atleast one image acquired by the second imaging device.
 11. The systemaccording to claim 9, wherein the pile is a tapered pile defining anouter surface that is inclined at a non-zero taper angle relative to thecentral axis of the pile, wherein the image processing system isconfigured to receive a predetermined value for the taper angle, and todetermine the attitude of the central axis of the pile based on thepositions and orientations of the border plane and the further borderplane, and on the predetermined value for the taper angle.
 12. A deviceconfigured to measure in real time an attitude of an object in a fixedframe of reference relative to earth from a moving frame of referencerelative to a vessel, the device comprising: an imaging deviceconfigured to acquire a plurality of images of the object at subsequenttimes in the moving frame of reference relative to the vessel; anattitude sensor configured to measure momentary attitudes for theimaging device with respect to the fixed frame of reference relative tothe earth during a period in which the plurality of images are acquiredand configured to generate attitude data, the attitude sensor having adynamic accuracy of better than 1.0 degrees, at least in pitch and rolldirections; a clock, arranged to time stamp the attitude data; and acommunication port configured to transmit data to a data processingsystem to determine in real time the attitude of the object in the fixedframe of reference relative to earth.
 13. The device according to claim12, wherein the data processing system is remote from the imaging deviceand the image data and attitude data is transmitted to the dataprocessing system.
 14. The device according to claim 12, wherein thedevice is portable and the imaging device and the attitude sensor arefixed to each other to prevent relative movement.
 15. The deviceaccording to claim 12, further comprising a global navigation satellitesystem device providing position data for the imaging device.
 16. Avessel comprising: a device configured to measure in real time anattitude of an object in a fixed frame of reference relative to earthfrom a moving frame of reference relative to a vessel, the devicecomprising: an imaging device configured to acquire a plurality ofimages of the object at subsequent times in the moving frame ofreference relative to the vessel; an attitude sensor configured tomeasure momentary attitudes for the imaging device with respect to thefixed frame of reference relative to earth during a period in which theplurality of images are acquired and configured to generate attitudedata, the attitude sensor having a dynamic accuracy of better than 1.0degrees, at least in pitch and roll directions; a clock, arranged totime stamp the attitude data; and a communication port configured totransmit data to a data processing system to determine in real time theattitude of the object in the fixed frame of reference relative toearth.
 17. A method of determining an attitude of an offshore object ina fixed frame of reference relative to earth based on observations froma vessel moving in a moving frame of reference relative to a vessel, themethod comprising: receiving, from a first imaging device, a pluralityof first images including the object acquired from the moving frame ofreference relative to the vessel in a first direction and at subsequenttimes to produce a first image data set; receiving, from a secondimaging device, a plurality of second images including the objectacquired from the moving frame of reference relative to the vessel in asecond direction not co-linear with the first direction and atsubsequent times to produce a second image data set; collecting, at anattitude sensing arrangement, attitude data representing a relativeattitude of the moving frame of reference relative to the vessel withrespect to the fixed frame of reference relative to the earth, during aperiod in which the first and second image data sets are acquired, foreach image data set; analyzing and combining, at the image processingsystem, the respective image data and attitude data to determine theattitude of the object in the fixed frame of reference relative toearth.
 18. The method according to claim 17, wherein the object is anoffshore pile and the attitude determines a verticality of the offshorepile during driving thereof.
 19. The method according to claim 17,wherein the object is a cable or pipe from a vessel and the attitudedetermines a departure trajectory of the cable or pipe from the vesselduring laying thereof.
 20. A non-transitory computer readable mediumstoring instructions which when executed by a processor, causes theprocessor to: receive, from a first imaging device, a plurality of firstimages including an object acquired from a moving frame of referencerelative to a vessel in a first direction and at subsequent times toproduce a first image data set; receive, from a second imaging device, aplurality of second images including the object acquired from the movingframe of reference in a second direction not co-linear with the firstdirection and at subsequent times to produce a second image data set;collect attitude data representing relative attitudes of the movingframe of reference relative to the vessel during a period in which thefirst and second image data sets are acquired with respect to a fixedframe of reference relative to the earth for each image data set; andanalyze and combine the respective image data sets and attitude data todetermine the attitude of the object in the fixed frame of referencerelative to earth.
 21. The non-transitory computer readable mediumaccording to claim 20, comprising further instructions, which whenexecuted by the processor causes the processor to: acquire time-stampedfirst and second image data sets; acquire time-stamped attitude data;determine a central plane or surface tangent plane of the object foreach of the first and second image data sets; combine attitude data withthe central planes or surface tangent planes for the first and secondimage data sets, for producing an estimate of the attitude; and displaythe estimated attitude.